Some electronic devices such as a notebook-sized personal computer or a tablet-type portable terminal are provided with a USB (Universal Serial Bus) interface, and it is possible to add functions to such an electronic device by connecting the electronic device with an external device, which is also provided with a USB interface. Such an external device may be selected from various devices including a printing device such as a printer, an imaging device such as a digital camera, a storage device such as a USB flash drive, a communication device such as a wireless LAN, and the like.
Regarding a USB, a USB standard specifies a power supply method to an external device in addition to a data transfer method. Accordingly, a USB provides the advantage that it is possible to supply electric power from an electronic device via a USB interface to an external device when the external device is not provided with any power source, and there is no need to separately prepare a power source as long as the external device is a low-power-consumption device such as a USB flash drive. A standard for a USB interface is, for example, the USB 2.0 standard or the USB 3.0 standard (see Japanese Registered Utility Model No. 3151486).
However, a USB standard limits a specification of current (electric power) available from an electronic device and therefore it is impossible to use an external device solely when an external device which needs current (electric power) exceeding the specification is connected with a USB interface of an electronic device for use. In such a case, there is a need to separately prepare a power source to supply necessary electric power to an external device.
Moreover, although a standard is extended from the USB 2.0 standard to the USB 3.0 standard and a specification of current is increased from 500 mA to 900 mA, there is still a need to separately prepare a power source when an external device which needs high electric power, such as a display, is used.